“Firefly,” which aired on Fox in 2002, was set in 2517 and revolved around the renegade crew of a spaceship.

Fans will see their favorite space cowboys reminisce about how it all began — from the crush Glau had on Fillion at auditions to Whedon’s first impressions to the show’s cancellation, paired with clips from the series.

Whedon went on to write “The Avengers,” Baccarin stars in the Emmy-winning “Homeland” and Fillion in the ABC hit “Castle.”

But Fillion says he gets as many fans coming up to him yelling “Capt’n” as “Castle.”

(His “Firefly” character was called “Cap’n Mal.”)

“The ‘Firefly’ fans, they’re with you for life. Joss Whedon fans are with you for life,” he says.

“Certainly a ‘Firefly’ fan will mention ‘Castle.’ ‘Oh, but we like ‘Castle,’ too!’ They’ll throw that in there for you — throw you the ‘Castle’ bone.”